Low resistivity silicon carbide

ABSTRACT

An opaque, low resistivity silicon carbide and a method of making the opaque, low resistivity silicon carbide. The opaque, low resistivity silicon carbide is doped with a sufficient amount of nitrogen to provide the desired properties of the silicon carbide. The opaque, low resistivity silicon carbide is a free-standing bulk material that may be machined to form furniture used for holding semi-conductor wafers during processing of the wafers. The opaque, low resistivity silicon carbide is opaque at wavelengths of light where semi-conductor wafers are processed. Such opaqueness provides for improved semi-conductor wafer manufacturing. Edge rings fashioned from the opaque, low resistivity silicon carbide can be employed in RTP chambers.

The present application is a continuation-in-part application of U.S.patent application Ser. No. 09/790,442 filed Feb. 21, 2001 which is anon-provisional application of Provisional Application Ser. No.60/184,766 filed Feb. 24, 2000.

BACKGROUND OF THE INVENTION

The present invention is directed to an improved low resistivity siliconcarbide. More specifically, the present invention is directed to animproved low resistivity silicon carbide that has a high nitrogenconcentration.

Silicon carbide (SiC), especially silicon carbide produced by chemicalvapor deposition (CVD), has unique properties that make it a material ofchoice in many high temperature applications. Chemical vapor depositionprocesses for producing free-standing silicon carbide articles involve areaction of vaporized or gaseous chemical precursors in the vicinity ofa substrate to result in silicon carbide depositing on the substrate.The deposition reaction is continued until the deposit reaches thedesired thickness. To be free standing, the silicon carbide is depositedto a thickness of upward of 0.1 mm. The deposit is then separated fromthe substrate as a free-standing article that may or may not be furtherprocessed by shaping, machining, or polishing and the like to provide afinal silicon carbide article.

In a chemical vapor deposition silicon carbide production run, a siliconcarbide precursor gas, such as a mixture of methyltrichlorosilane (MTS),hydrogen and argon, is fed to a deposition chamber where the mixture isheated to a temperature at which the mixture reacts to produce siliconcarbide. Hydrogen scavenges chlorine that is released from the MTS whenthe MTS dissociates during the reaction. An inert, non-reactive gas suchas argon or helium is employed as a carrier gas for MTS (a liquid atroom temperature). Inert gases also act as a diluent whose flow rate canbe varied to optimize the reaction and assures removal of by productsfrom the reaction/deposition zone. The silicon carbide deposits as alayer or shell on a solid mandrel provided in the deposition chamber.After the desired thickness of silicon carbide is deposited on themandrel, the coated mandrel is removed from the deposition chamber andthe deposit separated therefrom. The monolithic free-standing articlemay then be machined to a desired shape. Several CVD-SiC depositionsystems are described in U.S. Pat. Nos. 5,071,596; 5,354,580; and5,374,412.

Pure CVD-SiC has relatively high electrical resistivity. While this is adesirable characteristic for certain applications, such a characteristicis a limitation restricting its use in other applications. Certaincomponents, such as plasma screens, focus rings used in plasma etchingchambers need to be electrically conductive, and edge rings andsusceptors used in the RTP systems need to be opaque and possess hightemperature stability. While high temperature properties of CVD-SiC havemade it a material of choice for use in such chambers, its highresistivity has limited its use in fabricating components that require agreater degree of electrical conductivity.

High electrical resistivity of CVD-SiC has further restricted its use inapplications that are subject to the buildup of static electricity. Theneed to ground components used in such applications requires that theypossess greater electrical conductivity than is generally found inCVD-SiC. A low resistivity silicon carbide can provide a unique anduseful combination of high temperature properties with suitableelectrical conductivity properties for use in applications wheregrounding is required.

The semiconductor industry uses CVD-SiC components of differentelectrical resistivity. Different resistivity components providedifferent electrical coupling to plasma (i.e. absorbs different amountsof energy from the plasma) in plasma etch chambers. High resistivity SiCcomponents, i.e., 1000 to 5000 ohm-cm, such as gas diffuser plates, donot absorb much energy from the plasma and thus are used in those areasof the etch chamber where plasma energy is not affected. In contrast,low resistivity components such as plasma screens and liners absorbenergy from the plasma and thus prevent plasma from spreading beyond thecomponents. If the plasma is permitted to spread beyond the lowresistivity components, it will generate heat in the system which willdegrade the equipment.

Regardless of whether the CVD-SiC is of a high or low electricalresistivity, a uniform resistivity is always desired. A uniformelectrical resistivity provides for less variation in the performance ofthe SiC component whether it is functioning as a gas diffuser plate oras an edge ring. A SiC article with uniform resistivity also heats thecomponents uniformly, thus reducing the temperature gradients and thethermal stresses in the material. A uniform electrical resistivity of aSiC component is less susceptible to cracking in extreme environmentsemployed in manufacturing semiconductors such as in the extremeenvironments of rapid thermal processing (RTP).

U.S. patent application Ser. No. 09/790,442, filed Feb. 21, 2001,(non-provisional of provisionally filed U.S. application Ser. No.60/184,766, filed Feb. 24, 2000), assigned to the assignee of thepresent application discloses a chemical vapor deposited low resistivitysilicon carbide (CVD-LRSiC) and method of making the same. Electricalresistivity of the silicon carbide is 0.9 ohm-cm or less. In contrast,the electrical resistivity of relatively pure silicon carbide, prior tothe CVD-LRSiC of the application Ser. No. 09/790,442, is in excess of1000 ohm-cm. The method of preparing the CVD-LRSiC employs many of thesame components as the CVD methods disclosed above except that nitrogenis also employed. The lower resistivity of the silicon carbide isbelieved to be attributable to a controlled amount of nitrogenthroughout the silicon carbide as it is deposited by CVD. Controlledmeans that the nitrogen is maintained at a constant concentration duringSiC deposition. The nitrogen is incorporated in the deposit by providinga controlled amount of nitrogen with the precursor gas in the gaseousmixture that is fed to the reaction zone adjacent a substrate. Thereaction is carried out in an argon gas atmosphere. Nitrogenconcentration in the gaseous reaction mixture does not exceed 32% byweight of the mixture. At such nitrogen concentrations, the physicalproperties of SiC, other than the electrical resistivity, do not changesuch as to affect performance of SiC articles in thermal processingapplications. As the silicon carbide precursor reacts to form thesilicon carbide deposit, nitrogen from the gaseous mixture isincorporated into the deposit. The CVD-LRSiC contains at least 6.3×10¹⁸atoms of nitrogen per cubic centimeter of CVD-LRSiC. The electricalresistivity of the SiC was below 0.9 ohm-cm. Such low electricalresistivity is highly desirable and are suitable for use in thermalprocessing methods. However, the electrical resistivity of the SiC wasnot as uniform as the industry preferably desires. SiC samples rangeddown to 0.1 ohm-cm with a mean electrical resistivity of around 0.52ohm-cm with a variation around the mean of about 80%. Although such SiCis suitable for thermal processing methods, there is still a need for aSiC with a more uniform electrical resistivity and a lower variationaround the mean.

U.S. patent application Ser. No. 10/000,975 filed Oct. 24, 2001 alsoassigned to the assignee of the present application discloses anotherchemical vapor deposited low resistivity silicon carbide article. Thesilicon carbide has an improved electrical resistivity of less than 0.10ohm-cm and is opaque at wavelengths of from 0.1 μm to 1.0 μm at atemperature of at least 250° C. The improvements in the electricalresistivity as well as the opacity were achieved without compromisingother SiC properties such as thermal conductivity, flexural strength andthermal stability. The improved electrical resistivity and otherproperties were achieved by increasing the nitrogen incorporated intothe SiC. Nitrogen content reached 3×10¹⁹ atoms of nitrogen per cubiccentimeter of SiC. Nitrogen content was increased by increasing nitrogenvolume to as high as 50% in the deposition chamber. Surprisingly, theincrease in nitrogen concentration did not compromise the thermalconductivity or other important SiC properties. Altering reactioncomponents and chamber conditions may affect the stoichiometry of SiCreactants such that the resulting SiC may have reduced properties, suchas thermal conductivity, and may be prone to cracking. Thus, a workermay not readily predict the quality of SiC that he may obtain byaltering reaction parameters based on previous SiC synthesis processes.

While the resistivity of CVD-SiC can theoretically be lowered to adesired level by the introduction of a sufficient amount of impurities,such as boron, the resulting elevated levels of impurities adverselyaffect other properties of the SiC such as thermal conductivity and/orhigh temperature stability. The CVD-LRSiC is relatively free ofimpurities, containing less than 10 ppmw of impurity trace elements asdetermined by gas discharge mass spectroscopy. The CVD-LRSiC is furthercharacterized by a thermal conductivity of at least 195 Watts/meterdegree Kelvin (W/mK) and a flexural strength of at least 390 MPa.

The electrically conductive CVD-LRSiC possesses high temperaturestability in addition to being a high purity SiC. Thus, the freestanding CVD-LRSiC may be readily employed in high temperature furnacessuch as semiconductor processing furnaces and plasma etching apparatus.The CVD-LRSiC may be sold as a bulk material or may be further processedby shaping, machining, polishing and the like to provide a more finishedfree-standing article. For example, the CVD-LRSiC may be machined intoplasma screens, focus rings and susceptors or edge rings forsemi-conductor wafer processing and other types of high temperatureprocessing chamber furniture as well as other articles where CVD-LRSiCmaterial is highly desirable.

In the manufacture of semi-conductor wafers, there are numerous processsteps. One set of steps is referred to as epitaxial deposition, andgenerally consists of depositing a thin layer (from 10 to less than onemicron) of epitaxial silicon upon the wafer. This is achieved usingspecialized equipment such as SiC wafer boats or SiC susceptors or edgerings to secure the semi-conductor wafers in processing chambers, and achemical vapor deposition (CVD) process. The CVD process requires thatthe wafer be heated to very high temperatures, on the order of 1200° C.(2000° F.).

There has been a recent trend in the semi-conductor art to employequipment that operates upon a single wafer, rather than a group ofwafers. In single wafer equipment, the heating of the wafer to the CVDtemperature is greatly accelerated such that the wafer is taken fromabout room temperature to an elevated temperature within 30 seconds.Such processing is known as rapid thermal processing or RTP. Rapidthermal processing (RTP), for example, is used for several differentfabrication processes, including rapid thermal annealing (RTA), rapidthermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD),rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN).Temperatures in an RTP chamber may exceed 1100° C. and are subject torapid change, thereby making precise control of the substratetemperature complicated and difficult. RTP includes depositing variousthin films of different materials by an RTP-CVD process, rapid annealingof wafers (RTP thermal processing) and rapid oxidation to form silicondioxide. While the silicon wafer can accept such rapid temperaturechange well, the wafer must be held in position by a susceptor or edgering that can also withstand such rapid temperature changes. Susceptoror edge rings composed of CVD-SiC or CVD-LRSiC have proved very suitablefor withstanding RTP conditions.

Many RTP systems employ high intensity tungsten (W) halogen lamps toheat semi-conductor wafers. Pyrometers are used to measure and tocontrol wafer temperature by controlling the output of the W-halogenlamps. Accurate and repeatable temperature measurements for wafers overa wide range of values are imperative to provide quality wafers thatmeet the requirements for integrated circuit manufacturing. Accuratetemperature measurement requires accurate radiometric measurements ofwafer radiation. Background radiation from W-halogen lamps (filamenttemperature of 2500° C.) or from other sources can contribute to anerroneous temperature measurement by the pyrometer especially at lowtemperatures (400° C.) where the radiant emission from the wafer is verylow compared to the lamp output. Also, any light from the W-halogenlamps that passes through (transmits) a susceptor or edge ring can causean incorrect temperature reading by the pyrometer.

The industry has addressed the temperature problems by designing the RTPchamber with single sided heating and mounting the pyrometers on thechamber bottom opposite the light source. To further reduce lightinterference, the area under the wafer was made “light tight”, thuseliminating stray reflected light from entering the area under thewafer. In addition to redesigning the RTP chamber, CVD-SiC or CVD-LRSiCedge rings and susceptors were made more opaque to W-halogen lamp lightin the wavelength range that pyrometers operate by coating the ringswith about 200 μm (0.008 inches) of poly-silicon to reduce the lamplight that passes through the edge rings. Examples of such edge ringswhich are made more opaque to lamp light are disclosed in U.S. Pat. Nos.6,048,403 and 6,200,388 B1. However, although opaqueness may beincreased, coating edge rings with poly-silicon adds substantial cost tothe edge rings. Further, the coating process (epitaxial silicon growth)has many technical problems associated with it such as dendritic growth,bread loafing around edges and purity problems that reduce yields.Poly-silicon coating adds thermal mass to the edge rings. The increasedthermal mass limits heating ramp rates during RTP processing cycles.Ideally, edge rings have a thermal mass that is as low as possible toachieve the fastest heating ramp rates. The faster the ramp rate theshorter the processing cycle time for wafers, thus reducing waferprocessing costs. Another advantage to faster ramp rates is that thetotal integrated time at high temperature for the wafers is reducedallowing for less diffusion of any dopant species employed duringprocessing. Such is highly desirable as the feature sizes decrease forsemi-conductor devices (trend in the semi-conductor industry).

Although there are CVD-LRSiC articles that may be employed insemi-conductor wafer processing chambers, there is still a need forimproved CVD-LRSiC articles that have improved opaqueness and that donot need coatings to achieve a desired opaqueness. Additionally, thereis also a need for CVD-LRSiC articles that have more uniform electricalresistivity.

SUMMARY OF THE INVENTION

The present invention is directed to a silicon carbide that has anitrogen content greater than 3×10¹⁹ atoms/cm³. Advantageously, thesilicon carbide of the present invention has a resistivity of less than0.10 ohm-cm with a variation around an electrical resistivity mean ofless than 35%. Such a uniform low resistivity silicon carbide providesfor improved performance as furniture in thermal processing methods, andis less susceptible to cracking in extreme environments. Advantageously,the silicon carbide is opaque to light in a wavelength range of from 0.1μm to 1.0 μm. Silicon carbide of the present invention may be employedas furniture in semi-conductor processing chambers where accuratemaintenance of semi-conductor wafer temperatures are desired. Since thelow resistivity silicon carbide is opaque at wavelengths of light from0.1 μm to 1.0 μm, light from heating lamps employed in wafer processingchambers is significantly reduced, thus allowing a more accurate readingof wafer temperatures. Accordingly, defects in wafers caused by impropertemperatures are reduced or eliminated.

Further, because the low resistivity silicon carbide need not be coatedwith poly-silicon to provide the appropriate opaqueness, such problemsas dendritic growth, bread loafing and the high cost of coating areeliminated. Also, the problem of adding thermal mass to silicon carbidearticles is eliminated. Thus, faster heating ramp rates for rapidthermal processing are achieved with articles prepared from the lowresistivity silicon carbide of the present invention. The faster heatingramp rates in turn provide for reduced total integrated time at hightemperatures for wafers and reduced diffusion of dopant duringprocessing.

The present invention is also directed to a method of making the opaque,low resistivity silicon carbide. The silicon carbide of the presentinvention is prepared by chemical vapor deposition. Gas phase nitrogenconcentrations of at least 56% by volume are employed in the CVDprocess. Reactants are mixed together with the high concentrations ofnitrogen in a CVD chamber, and the silicon carbide product is depositedon a substrate such as a mandrel. Total furnace pressure during the CVDprocess is at least 300 torr. Surprisingly, by increasing total furnacepressure to at least 300 torr to increase nitrogen concentration,desirable properties such as thermal conductivity, thermal stability andthe like are not compromised. Accordingly, the silicon carbide articlesmay readily be employed CVD and RTP processes.

In addition to having low resistivity and to being opaque to light atwavelengths of from 0.1 μm to 1.0 μm, the opaque, low resistivitysilicon carbide is stable at high temperatures, has a high thermalconductivity and is of a high purity. The opaque, low resistivitysilicon carbide may be removed from the mandrel and retained in bulkform or may be shaped, machined and polished to provide a final article.

A primary objective of the present invention is to provide for a freestanding, low resistivity silicon carbide with a nitrogen contentgreater than of 3×10¹⁹ atoms/cm³.

Another objective of the present invention is to provide for a freestanding, low resistivity silicon carbide that is opaque to light atwavelengths of from 0.1 μm to 1.0 μm.

A further objective of the present invention is to provide a thermallystable free standing, low resistivity silicon carbide.

An additional objective of the present invention is to provide a freestanding, low resistivity silicon carbide that has a high purity.

Still yet, a further objective of the present invention is to provide amethod of making an opaque, low resistivity silicon carbide.

Other objectives and advantages of the present invention may beascertained by a person of skill in the art after reading the followingdisclosure and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a CVD apparatus that may be used to fabricateopaque, low resistivity silicon carbide;

FIG. 2A is a top view of an edge ring composed of CVD deposited opaque,low resistivity silicon carbide; and

FIG. 2B is a cross-section of an edge ring composed of CVD depositedopaque, low resistivity silicon carbide.

DETAILED DESCRIPTION OF THE INVENTION

Chemical vapor deposited (CVD) silicon carbide (SiC) of the presentinvention has a nitrogen content of greater than 3×10¹⁹ atoms/cm³. Thesilicon carbide has low electrical resisitivity of less than 0.10ohm-cm, and is opaque to light at wavelengths of from 0.1 μm to 1.0 μm.Such silicon carbide is bulk or free-standing, i.e., self-supporting.Bulk or free-standing silicon carbide is distinguished from thin filmsilicon carbide which is deposited upon a surface with the intent thatthe silicon carbide remain permanently bonded to the surface. Bulk orfree-standing silicon carbide can be machined and polished to a desiredsize and shape. Such silicon carbide may be employed as furniture inprocessing chambers for chemical vapor deposition (CVD), rapid thermalprocessing (RTP), epitaxial deposition and the like. In addition tohaving a low resistivity and being opaque to light at wavelengths offrom 0.1 μm to 1.0 μm, the silicon carbide has a high thermalconductivity, high flexural strength, and high thermal stability. Theopaque, low resistivity silicon carbide is substantially free ofmetallic impurities containing less than about 3 ppmw of trace elementssuch as boron, phosphorous and the like as determined by gas dischargemass spectroscopy. Additionally, the opaque, low resistivity siliconcarbide has reduced thermal mass.

The opaque, low resistivity silicon carbide is prepared by chemicalvapor deposition. To provide a substantially opaque, low resistivitysilicon carbide, high concentrations of nitrogen are incorporated intothe CVD silicon carbide. High concentrations of nitrogen areincorporated into the silicon carbide by providing a high concentrationnitrogen atmosphere for the silicon carbide precursors to react.Nitrogen atmosphere means that no inert, non-reactive gases such asargon, helium or other noble gas are employed in preparing the opaque,low resistivity silicon carbide. The atmosphere composes at least 56% byvolume of nitrogen, preferably from 56% to 90% by volume of nitrogen.More preferably the atmosphere composes from 60% to 90% by volume ofnitrogen to obtain the desired nitrogen content and electricalresistivity of silicon carbide. The remainder of the atmosphere iscomposed of hydrogen gas and silicon carbide precursors. Nitrogen isincorporated into silicon carbide in amounts of greater than 3×10¹⁹atoms of nitrogen per cubic centimeter of silicon carbide. Preferably,nitrogen is incorporated into the silicon carbide in amounts of from5×10¹⁹ to 1×10²⁰ atoms/cm³, more preferably from 7×10¹⁹ to 1×10²⁰atoms/cm³. While not being bound to theory, nitrogen is believed to actas a dopant that reduces band gaps in the silicon carbide to reduceelectrical resistivity. Surprisingly, raising the percentage orconcentration of nitrogen to 56% by volume or greater increases theamount of nitrogen incorporated into the silicon carbide withoutnitrogen saturation. Further, the amount of nitrogen incorporation from3×10¹⁹ atoms/cm³ to 1×10²⁰ atoms/cm³ unexpectedly provides for bothdecreased electrical resistivity and a more uniform electricalresistivity. Nitrogen may be employed in any suitable form such asN_(2(g)), volatile organic compounds containing —NO₂ or amine groupssuch as —NH₂, compounds of —N(H)₄ ⁺ and quaternary amines, NO₃ ⁻ saltsin aqueous form, halogen containing nitrogen compounds, and the like.Examples of suitable nitrogen compounds include NF₃ and NH₃.

Silicon carbide precursor is selected from any suitable material thatcan be reacted to form silicon carbide. Such materials include acomponent, such as a silane or a chlorosilane, which can react to form asilicon moiety and a component such as a hydrocarbon that can react toform a carbon moiety. The component contributing the silicon moiety canbe different from, or can be the same as, the component contributing thecarbon moiety. Hydrocarbon substituted silanes are preferred siliconcarbide precursors because they contain both the silicon carbidemoieties in a single compound. The precursor(s) or reactant(s) can be acompound which dissociates at the reaction conditions forming one orboth of the silicon carbide moieties, or the precursors can be two ormore compounds which react to provide one or both of the moieties. Whilethe precursor(s) needs to be in the gas phase when reacted in thevicinity of the substrate, it is not necessary that the precursor'sboiling point be less than ambient temperature. Methyltrichlorosilane(MTS) is a preferred precursor, especially when used with hydrogen (H₂),which scavenges chlorine released when MTS dissociates. Since MTSprovides both silicon and carbon in a stoichiometric ratio of 1:1, noother source of silicon and carbon moieties is required. Silicon carbideof the present invention may be prepared by any suitable method in theart provided that a nitrogen atmosphere in the reaction chamber is atleast 56% by volume. H₂/MTS molar ratios may range from 2 to 10,preferably from 4 to 10. Hydrogen partial pressures may range from 92torr to 130 torr, preferably 100 torr to 115 torr. MTS partial pressuresmay range from 16 torr to 30 torr, preferably from 20 torr to 25 torr.

Nitrogen may be provided in any suitable form as long as the form issufficiently volatile or may be made sufficiently volatile to form agas. Nitrogen partial pressures may range from 191 torr to 675 torr,preferably from 200 torr to 560 torr. No inert, non-reactive gases suchas argon, helium or other noble gas are employed in preparing theopaque, low resisitivity silicon carbide.

Total deposition chamber or furnace pressures range from 300 torr to 835torr, preferably from 320 torr to 700 torr. The furnace pressures equalthe sum of the partial pressures of the reactants. Such high totalfurnace pressures provide for the desired nitrogen concentrations in thesilicon carbide. Deposition chamber temperatures range from 1250° C. to1400° C., preferably from 1300° C. to 1375° C.

Flow rates may vary depending upon the specific apparatus employed.Generally, the flow rate of MTS ranges from 15 slpm to 50 slpm. The flowrate of hydrogen ranges from 25 slpm to 150 slpm, and the flow rate ofnitrogen ranges from 70 slpm to 700 slpm.

Deposition substrates may be composed of any suitable material that canwithstand the harsh conditions of the deposition chambers. An example ofa suitable substrate or mandrel for depositing silicon carbide isgraphite. Graphite mandrels as well as other types of mandrels may becoated with a release agent such that deposited silicon carbide may bereadily removed from the mandrel after deposition. Silicon carbidedeposits may be removed from mandrels by controlled oxidation(controlled combustion).

An example of a chemical vapor deposition system for producing opaque,low resistivity silicon carbide articles of the present invention isillustrated in FIG. 1. Deposition is carried out within furnace 10. Astainless steel wall provides a cylindrical deposition chamber 12.Heating is provided by a graphite heating element 14 which is connectedto an external power supply by an electrode 16. Graphite depositionmandrels are arranged within a graphite isolation tube 20 and gas isintroduced by means of an injector 22 through the upper end of theisolation tube so that the reaction gases sweep along mandrels 18. Oneor more baffle(s) 24 is used to control the aerodynamics of gas flowthrough furnace 10.

Line 26, which supplies injector 22, is fed by a nitrogen cylinder 28, ahydrogen cylinder 30, and a MTS bubbler 32. Nitrogen is fed by lines 34and 36 both directly to inlet line 26 and through bubbler 32. Thehydrogen cylinder 30 is connected by line 38 to the inlet line 26.Nitrogen flow through lines 32 and 36 and hydrogen flow through line 38are controlled by mass flow controllers 40, 42, and 44. The MTS bubbler32 is maintained at a constant temperature by a constant temperaturebath 46. A pressure gauge 48 is connected to a feed back loop thatcontrols the gas pressure of bubbler 32.

Outlet line 50 is connected to a bottom outlet port 51. Pressure withinthe deposition chamber 12 is controlled by a vacuum pump 52 which pullsgases through the chamber and a furnace pressure control valve 54operably connected to the vacuum pump. Temperature and pressure withindeposition chamber 12 are measured by thermal probe 58 and pressureindicator 56. Exhaust gases are passed through filter 60 to removeparticulate material upstream of the pressure control valve and througha scrubber 62 downstream of the vacuum pump to remove HCl. Otherexamples of such processes and the apparatus employed are described inU.S. Pat. Nos. 5,354,580. 5,374,412 and 5,474,613, the disclosures ofwhich are hereby incorporated herein in their entireties by reference.

After deposition the bulk or free-standing silicon carbide may be soldin bulk form or further processed by shaping, machining, polishing andthe like to form a desired article. Further processing involves numerousmethods that are well known in the art for free-standing siliconcarbide. Such methods often involve diamond polishing and machining. Anexample of an article made from the silicon carbide is a susceptor oredge ring employed to hold or secure semi-conductor wafers forprocessing in furnaces or other suitable chambers.

Advantageously, the free-standing low resistivity silicon carbidebecomes opaque at light wavelengths of from 0.1 μm to 1.0 μm, typicallyfrom 0.7 μm to 0.95 μm. Low resistivity silicon carbide may remainopaque to light at wavelengths of from 0.1 μm to 1.0 μm at temperaturesof from 20° C. to temperatures of up to 1475° C., typically from 300° C.to 1300° C. Opaque means that light transmitted through the siliconcarbide has an absorption coefficient that ranges from 259 cm⁻¹ to 1000cm⁻¹, typically from 270 cm⁻¹ to 800 cm⁻¹. Thickness of such opaque, lowresistivity silicon carbide ranges from 0.05 mm to 2.00 mm, typicallyfrom 0.1 mm to 1.0 mm. At such wavelengths, silicon carbide articles donot substantially transmit light and are highly suitable for furniturein semi-conductor processing chambers. Such chambers include, but arenot limited to, RTP processing chambers where pyrometers that operate atwavelengths of 0.7 μm to 0.95 μm monitor semi-conductor wafertemperatures. Such RTP chambers may operate at temperatures of from 300°C. to 1250° C. Opaque low resistivity silicon carbide of the presentinvention remains opaque to light at wavelengths of from 0.1 μm to 1.0μm at such temperatures. Since the opaque, low resistivity siliconcarbide does not substantially transmit light at wavelengths wherepyrometers operate, temperature readings of semi-conductor wafers aremore accurate when processed on furniture composed of the siliconcarbide of the present invention. Thus, fewer defects occur in processedwafers and both manufacturing and cost efficiency are improved.

Since the low resistivity silicon carbide is opaque to light wavelengthsat which a pyrometer operates, susceptors or edge rings employed in RTPneed not be coated with poly-silicon. Accordingly, articles composed ofsilicon carbide within the scope of the present invention eliminate theproblems of epitaxial silicon growth such as dendritic growth, breadloafing and any impurities that may occur during the poly-siliconcoating process. Such impurities may lead to contamination ofsemi-conductor wafers during RTP. Elimination of the poly-siliconcoating process also reduces over-all cost of making silicon carbidearticles as well as processing semi-conductor wafers.

Additionally, elimination of unnecessary coatings on edge rings reducesthe thermal mass of the edge rings and further improves the RTP insemi-conductor wafer manufacturing. Reduced thermal mass permits fasterramp rates in RTP, thus reducing semi-conductor processing cycle time,and reducing wafer processing costs. Further, as ramp rates increase thetotal integrated time at high temperatures for wafers is reducedallowing for less diffusion of any dopant species in the RTP. Reduceddiffusion of dopant species into semi-conductor wafers in turn permitsfeatures on the wafers to be decreased in size (an industry goal).

Low electrical resistivity of the silicon carbide of the presentinvention is also desirable for radio frequency (RF) heated susceptorsto couple the RF field to the susceptor and for components in the plasmaetch chamber to couple the plasma energy to the component. In addition,due to low resistivity the wafer holders can be grounded and do notbuild a static charge on them. Articles made from the low resistivitysilicon carbide include, but are not limited to, edge rings or susceptorrings, wafer boats, epi susceptors, plasma etch components, focusedrings, plasma screens and plasma chamber walls, and the like. Becausethe silicon carbide of the present invention has a low resistivity, thesilicon carbide may be employed as components in electrical devices suchas electrodes and heating elements. In addition to having a lowelectrical resistivity, the electrical resistivity is more uniform.Chemically vapor deposited low electrical resistivity silicon carbideprepared by the method of the present invention may have an electricalresistivity of less than 0.10 ohm-cm. Preferred silicon carbide preparedby the method of the present invention has an electrical resistivity offrom 0.01 ohm-cm to 0.08 ohm-cm. The average value for the electricalresistivity is from 0.02 ohm-cm to 0.06 ohm-cm with an electricalresistivity variance around the mean of less than 35%, typically from25% to 30%.

FIGS. 2A and 2B show an edge ring machined and polished from a singlepiece of opaque, low resistivity silicon carbide within the scope of thepresent invention. Edge ring 100 has a circular circumference composedof main ring component 102 with wafer holding flange 104 continuous withmain ring component 102. Wafer holding flange 104 terminates at edge 106to form void 108. FIG. 2B is a cross-section of edge ring 100 along lineA—A. Main ring component 102 terminates at an outer surface in supportflange 110 that is continuous with main ring component 102. Main ringcomponent 102 terminates at an inner surface with flange 112 which iscontinuous with wafer holding flange 104, thus securing wafer holdingflange 104 to main ring component 102.

Although FIGS. 2A and 2B show an edge ring as a circular article, anysuitable shape may be employed. A circular edge ring is illustrated inFIGS. 2A and 2B because many semi-conductor wafers are in a circularshape. Size and thickness of the edge ring may also vary. However, thethinner the edge ring, the more suitable the ring is for processingsemi-conductor wafers. A thin edge ring can heat up faster in waferprocessing furnaces than a relatively thick edge ring, thus reducing theamount of processing time. Thickness of opaque low resistivity siliconcarbide edge rings may range from 0.1 mm to 1.0 mm, preferably from 0.25mm to 0.5 mm.

The following examples are intended to further illustrate the presentinvention and are not intended to limit the scope of the invention.

EXAMPLE 1

An opaque, low resistivity silicon carbide was produced according to amethod of the present invention in an apparatus similar to thatillustrated in FIG. 1 with the following process conditions: depositiontemperature=1350° C., total furnace pressure=300 torr, hydrogen partialpressure=92 torr, MTS partial pressure=17 torr, and nitrogen partialpressure=191 torr. The nitrogen partial pressure corresponded to 63.6%of nitrogen in the process. One inch equals 2.54 centimeters. Sixresistivity samples were fabricated from different locations in oneplate (24×29-inches). The resistivity values fell in the range of0.057-0.071 ohm-cm. The average electrical resistivity of the sixsamples was 0.064 ohm-cm. The deposition thickness range was0.24-0.564-inches yielding a deposition rate of 2.7-6.3 mil/hour. Thesilicon carbide was characterized for thermal conductivity by laserflash technique, solid phase N₂ concentration by secondary ion massspectroscopy, and purity by gas discharge mass spectroscopy. The resultsare given below:

Property Value Thermal conductivity 232-257 W/mK Solid phase N₂ conc.3-4.6 × 10¹⁹ atoms/cc Chemical impurity 1.14 (ppmw)The above property values showed that large amounts of N₂ incorporationdid not significantly affect the important properties of siliconcarbide. The thermal conductivity was high and exceeded 200 W/mK. Thechemical impurities were less than 2 ppmw. Further, electricalresistivity was below 0.1 ohm-cm. Thus, the silicon carbide had reducedelectrical resistivity values.

EXAMPLE 2

An opaque, low resistivity silicon carbide was produced according to amethod of the present invention in an apparatus similar to thatillustrated in FIG. 1 with the following process conditions: depositiontemperature=1350° C., total furnace pressure=400 torr, hydrogen partialpressure=100 torr, MTS partial pressure=16 torr, and nitrogen partialpressure=284 torr. The nitrogen partial pressure corresponded to 72% ofnitrogen in the process. Six resistivity samples were fabricated fromdifferent locations in one plate (24×29-inches). The resistivity valuesfell in the range of 0.04-0.043 ohm-cm. The average resistivity of thesix samples was 0.041 ohm-cm. The deposition thickness range for 78 hourdeposition was 0.172-0.355-inches yielding a deposition rate of 2.2-4.5mil/hour. The material was characterized as in Example 1 and the resultsare given below:

Property Value Thermal conductivity 227-283 W/mK Solid phase N₂ conc.4.2 × 10¹⁹ atoms/cc Chemical impurity 1.85 (ppmw)The above property values of CVD-SiC showed that large amounts of N₂incorporation did not significantly affect the important properties ofsilicon carbide. As in Example 1, thermal conductivity was high andexceeded 200 W/mK. Chemical impurities were below 2.

EXAMPLE 3

An opaque, low resistivity silicon carbide was produced according to amethod of the present invention in an apparatus similar to thatillustrated in FIG. 1 with the following process conditions: depositiontemperature=1350° C., total furnace pressure=650 torr, hydrogen partialpressure=110 torr, MTS partial pressure=17 torr, and nitrogen partialpressure=523 torr. The nitrogen partial pressure corresponds to 80% ofnitrogen in the process. Six resistivity samples were fabricated fromdifferent locations in one plate (24×29-inches). The resistivity valuesfell in the range of 0.015-0.028 ohm-cm. The average resistivity was0.021 ohm-cm. The deposition thickness range for 84 hours of depositionwas 0.134-0.302-inches yielding a deposition rate of 1.6-3.6 mil/hour.The material was characterized as in Example 1 and the results are givenbelow:

Property Value Thermal conductivity 220-257 W/mK Solid phase N₂ conc.7.6 × 10¹⁹ atoms/cc Chemical impurity 2.07 (ppmw)The above property values of CVD-SiC showed that large amounts of N₂incorporation did not significantly affect the important properties ofsilicon carbide. As in Examples 1 and 2 above, silicon carbide madeaccording to a method of the present invention had electricalresistivity values of less than 0.1 ohm-cm and maintained high thermalconductivity values as well as low impurities.

EXAMPLE 4

Four silicon carbide plates were prepared to show that the method of thepresent invention provides for opaque silicon carbide articles in thelight wavelength range of from about 0.10 μm to about 1.0 μm. Two plateswhich acted as controls had a thickness of 0.0050 inches. Two platesthat were prepared according to the method of the present invention hada thickness of 0.0055 inches.

The two plates that were prepared according to the method of the presentinvention were prepared according to the method described in Example 1above except that deposition was stopped when the desired thickness ofeach plate was achieved.

The two control plates were prepared by chemical vapor deposition in a1.5-m production furnace of Morton Advanced Materials, Woburn, Mass.,with the following process conditions: deposition temperature=1350° C.,total pressure=200 torr, hydrogen partial pressure=90 torr, MTS partialpressure=17 torr, and nitrogen partial pressure=93 torr. Deposition ratewas 1.5 microns/minute.

After each test sample was prepared, each sample was mounted in a holderwhich was placed in front of a laser beam of appropriate wavelength.Opaqueness of each sample was determined by measuring each samples lighttransmission at a wavelength of 0.8 microns and 1.06 microns. A detectorsilicon was placed near the backside of each sample to measure thetransmitted signal from the light source. The light source for the 0.8microns wavelength was a titanium/sapphire laser, and the light sourcefor the 1.06 microns wavelength was an yttrium/neodymium laser. Spotdiameter on each sample was 4 mm. Incidence angle was 0° and pulse ratewas 10 Hz. Each sample was exposed to 200 pulses of light at therespective wavelengths. The tests were done at room temperature (20°C.).

From the transmission values, the absorption coefficient was calculatedfor each sample taking the refractive index of SiC as 2.58 at 1.06micron and 2.61 at 0.8 micron. The following formula was used tocalculate the absorption coefficient:Absorption Coefficient=−(1/t)ln{[((1−R)⁴/4R ⁴ T ²)+(1/R ²)]^(0.5)−((1−R)²/2R ² T)}

Where t is sample thickness, T is fraction of light transmitted, R isfraction of light reflected which is calculated as follows:

 R=[(1−n)²/(1+n)²]

The results of the test and calculations are given in the table below.

Absorption Absorption Sample Transmission Coefficient at TransmissionCoefficient at Sample Thickness at 1.06 1.06 micron at 0.8 microns 0.8micron Number (inch) microns (%) (cm⁻¹) (%) (cm⁻¹) Control 1 0.00500.279 429 5.13 199 Control 2 0.0050 0.476 387 4.21 215 Opaque 1 0.00550.038 533 1.72 259 Opaque 2 0.0055 0.052 510 1.2 285

The data showed that the amount of light transmission in the samplesprepared by the method of the present invention (opaque 1 and 2) hadreduced light transmissions and higher absorption coefficient at bothwavelengths. In contrast, the controls which were not prepared by dopingsilicon carbide with as much nitrogen had higher transmissions and lowerabsorption coefficients at both wavelengths. Accordingly, the siliconcarbide of the present invention had improved opacity over siliconcarbide prepared by the lower nitrogen doping process.

1. A free-standing article of CVD silicon carbide comprising greaterthan 3×10¹⁹ atoms of nitrogen/cm³ and an electrical resistivity of lessthan 0.10 ohm-cm, the free standing article of CVD silicon carbide ismade from precursors at temperatures from 1250° C. to 1400° C.
 2. Thearticle of claim 1, wherein the CVD silicon carbide has from 5×10¹⁹ to1×10²⁰ nitrogen atoms/cm³.
 3. The article of claim 1, wherein the CVDsilicon carbide has from 7×10¹⁹ to 1×10²⁰ nitrogen atoms/cm³.
 4. Thearticle of claim 1, wherein the CVD silicon carbide has an electricalresistivity of from 0.10 ohm-cm to 0.08 ohm-cm.
 5. The article of claim1, wherein the CVD silicon carbide has an average electrical resistivityis from 0.02 ohm-cm to 0.06 ohm-cm.
 6. The article of claim 1, whereinthe CVD silicon carbide is opaque to light at a wavelength of from 0.1μm to 1.0 μm.
 7. The article of claim 1, wherein the CVD silicon carbidearticle is an edge ring, plasma screen or liner.
 8. The article of claim1, wherein the CVD silicon carbide has an absorption coefficient of from259 cm⁻¹ to 1000 cm⁻¹.
 9. The article of claim 1, wherein the variationaround the mean of the electrical resistivity is from 25% to 30%. 10.The article of claim 6, wherein the article is opaque to the lighttemperatures of from 20° C. to 1475° C.